1. Field of the Invention
The present invention relates to a rare earth alloy ingot, a sintered magnet comprising the rare earth alloy ingot, a production method for a rare earth alloy ingot, a production method for a rare earth alloy flake, an R-T-B type magnet alloy ingot, an R-T-B type magnet, an R-T-B type magnet alloy flake, an R-T-B type magnet alloy powder, an R-T-B type bonded magnet, an R-T-B type exchange spring magnet alloy ingot, an R-T-B type exchange spring magnet, an R-T-B type exchange spring magnet alloy powder, and an R-T-B type exchange spring bonded magnet.
2. Description of the Related Art
In recent years, production of Nd—Fe—B alloys serving as magnet alloys has sharply increased by virtue of high-performance characteristics of the alloys, and these alloys are employed in HDs (hard disks), MRI (magnetic resonance imaging), a variety of motors, etc. Typically, a portion of Nd atoms is substituted by another rare earth element such as Pr or Dy (as used herein, Nd and the substituted Nd are referred to as R, moreover, Y is at least one selected from the rare earth elements containing Y) or a portion of Fe is substituted by another metal element such as Co, Ni, Cu, Ga or Al (as used herein, Fe and the substituted Fe are referred to as T). Such substituted alloys as well as Nd—Fe—B alloys are generally referred to as R-T-B type alloys.
An R-T-B alloy contains, as the dominant phase, a ferromagnetic phase formed of R2T14B crystals, which contribute to magnetization, and, in grain boundaries of the T2T14B crystals, a nonmagnetic R-rich phase having a low melting point and containing a rare earth element(s) at high concentration. The R-T-B alloy is an active metallic material. Therefore, the alloy is generally melted and mold-cast in vacuum or under inert gas.
In a typical method of producing a magnet, an ingot of the alloy is pulverized to powder having a particle size of about 3 μm (as measured by means of FSSS (Fisher Sub-Sieve Sizer)); the powder is subjected to press-forming in a magnetic field; the resultant compact is sintered in a sintering furnace at a temperature as high as about 1,000 to about 1,100° C.; and in accordance with needs, the sintered product is heated, mechanically processed, and plated for corrosion prevention.
The R-rich phase plays the following important roles.    (1) Since the R-rich phase has a low melting point, the phase liquefies during sintering, thereby contributing to achievement of high density of the resultant magnet, leading to improved magnetization.    (2) The R-rich phase functions to smoothen grain boundaries, thereby reducing the number of nucleation sites in a reversed magnetic domain, thereby enhancing the coercivity.    (3) The R-rich phase magnetically insulates the dominant phase, thereby enhancing the coercivity.
Thus, attainment of a uniformly dispersed R-rich phase is critical, because otherwise magnet characteristics of the produced magnet are adversely affected.
The distribution of the R-rich phase in a magnet—the final product—depends greatly on the metallographic structure of the raw material alloy ingot. Specifically, when the alloy is mold-cast, a slow cooling rate often results in formation of large crystal grains. In such a case, the particle size of the pulverized product becomes considerably smaller than that of the crystal grain size. When the alloy is mold-cast, R-rich phase is included not in crystal grains but virtually in crystal gain boundaries. Therefore, particles formed only of the dominant phase containing no R-rich phase and those formed only of the R-rich phase result, making it difficult to mix the dominant phase and R-rich phase homogeneously.
Another problem involved in mold casting is that γ-Fe tends to be formed as primary crystals, due to the slow cooling rate. At approximately 910° C. or lower, γ-Fe transforms into α-Fe, which deteriorates pulverization efficiency during production of magnets. If α-Fe remains even after sintering, magnetic characteristics of the sintered product are deteriorated. Thus, the ingot obtained through mold casting must be subjected to homogenization treatment at high temperature for a long period of time in order to remove α-Fe.
In order to solve the above problems, the strip casting method (hereinafter referred to as the SC method), which ensures a cooling rate during casting faster than that attainable by mold casting, is proposed and employed in actual production steps.
In the SC method, a molten metal is slowly poured onto a copper roll whose inside is cooled by water and which rotates at a peripheral velocity of about 1 mm/sec, and is solidified through rapid cooling, to thereby produce flake having a thickness in a range from about 0.1 to 1 mm (Japanese Patent Application Laid-Open (kokai) Nos. 05-222488 and 05-295490). During casting, the molten metal was solidified through rapid cooling, to thereby yield an alloy having a microcrystalline structure in which R-rich phase is minutely dispersed. Since the R-rich phase is minutely dispersed in the alloy, dispersion of R-rich phase in the product obtained by pulverizing and sintering the alloy becomes also satisfactory, to thereby successfully attain improved magnetic characteristics (Japanese Patent Application Laid-Open (kokai) Nos. 5-222488 and 5-295490). However, even when the above method is employed, α-Fe is unavoidably formed as the R content (%) decreases. For example, when the Nd content of an Nd—Fe—B ternary alloy is 28% by weight or less, α-Fe generation becomes significant.
The thus-formed α-Fe considerably deteriorates pulverizability of an alloy ingot in magnet production steps.
FIG. 9 is a back-scattered electron image, observed under an SEM (scanning electron microscope), showing a cross section of an Nd—Fe—B ingot (Nd: 30.0% by mass) cast through a conventional SC method.
In FIG. 9, Nd-rich (i.e., R of the R-rich phase is Nd) phase corresponds to bright portions. Some portion of the Nd-rich phase assumes the shape of linked rods extending in the solidification direction (left (roll side) to right (free side)). Another portion of the Nd-rich phase assumes a dot shape and is dispersed. In the rod-shaped Nd-rich phase, the growth direction in grain boundaries and that in crystal grains coincide with the longitudinal direction of the rod-shaped Nd-rich phase. Although the rod-shaped phase is slightly reduced or fragmented through heat treatment performed after cast, effects exerted during casting still prevail, and the dot-shaped or rod-shaped Nd-rich phase shows nonuniform dispersion. Such a microcrystalline feature is typical to a cross-sectional metallographic structure of an Nd—Fe—B alloy ingot cast through the SC method.
As explained above, the R-T-B type alloy contains, as the dominant phase, a ferromagnetic phase formed of R2T14B crystals, which contribute to magnetization, and, in grain boundaries of the R2T14B crystals, a nonmagnetic R-rich phase having a low melting point and containing a rare earth element(s) at high concentration. The R-T-B type alloy is an active metallic material. Therefore, the alloy is generally melted and cast in vacuum or under inert gas, and the cast alloy provides sintered magnets and bonded magnets. Below, the sintered magnet and the bonded magnets are explained.
(1) Sintered Magnet
Alloy ingots for sintered magnets are produced through, among other methods, the book molding method (hereinafter referred to as the BM method) and the SC method. In the BM method, a molten metal is cast in a copper mold or an iron mold whose inside is cooled by water, to thereby produce an ingot having a thickness of about 5 to about 50 mm.
The alloy ingot produced through any of the above methods is pulverized in an inert gas atmosphere, such as argon, nitrogen, to have a particle size of about 3 μm (as measured by means of an FSSS (Fisher Sub-Sieve Sizer)); the resultant powder is subjected to press-forming in a magnetic field at 0.8 to 2 ton/cm2; the resultant compact is sintered in a sintering furnace at a temperature as high as about 1,000 to about 1,100° C. (hereinafter, the steps of pulverization to sintering are collectively referred to as the powder metallurgical method); and in accordance with needs, the sintered product is heated at 500 to 800° C., mechanically processed, and plated for prevention of corrosion, to thereby produce a magnet.
Among these methods, the SC method provides a minute microcrystalline structure and forms an alloy in which a low-melting temperature R-rich phase formed of concentrated nonmagnetic rare earth elements is minutely dispersed. Since the R-rich phase is minutely dispersed in the alloy, dispersibility of the R-rich phase after pulverizing and sintering the alloy also becomes satisfactory, to thereby successfully attain improved magnetic characteristics as compared with those of alloy ingots produced through the BM method.
(2) Bonded Magnet
An alloy ingot for bonded magnets, in the form of ribbon having a thickness in a range from 10 to 100 μm, is produced through the ultra-rapid-cooling method; i.e., by injecting a molten metal from a crucible, via an orifice provided in the bottom of the crucible, onto a copper roll which rotates at a high peripheral velocity of about 20 m/sec. The ribbon produced through the ultra-rapid-cooling method may be heated at 400 to 1,000° C. in accordance with needs, followed by pulverization to powder having a particle size of 500 μm or less. A mixture of the powder and a resin is press-molded or injection-molded, to thereby form a magnet. Since the ribbon is isotropic in terms of magnetic characteristics, the bonded magnet produced from the ribbon also exhibits magnetic isotropy.
Recently, there has been proposed an exchange spring magnet having a composite structure of a hard magnetic phase and a soft magnetic phase, each phase comprising crystal grains in a range from 10 to 100 nm in size. An alloy ingot for exchange spring magnets, containing considerably minute crystal grains, is generally produced through the ultra-rapid-cooling method. The produced ingot may be heated at 400 to 1,000° C. in accordance with needs, followed by pulverization to powder having a particle size of 500 μm or less. A mixture of the powder and a resin is press-molded or injection-molded, to thereby form an exchange spring magnet. In the exchange spring magnet, residual magnetic flux density and coercive force are generally determined by crystal grains of the soft magnetic phase and crystal grains of the hard magnetic phase, respectively. Since the hard magnetic phase of the exchange spring magnet must exhibit a highly anisotropic magnetic field, the hard magnetic phase is formed of a rare earth material such as R2T14B, Sm1Co5, or Sm2Co17. The soft magnetic phase is formed of Fe, Fe2B, Fe3B, etc., which exhibit high saturation magnetization.
In an as-cast state, R-T-B type magnet alloy ingots produced through the BM method or the SC method exhibit very weak magnetic characteristics, and so, they cannot be used as a magnet. The reason therefor is as follows. In the case of R-T-B type magnets, coercive force is exhibited on a nucleation-based mechanism. Specifically, crystal grain boundaries contain lattice defects and irregularities in an as-cast state, and these lattice defects and irregularities serve as nuclei for generating a reverse magnetic domain (hereinafter the nuclei are referred to as nucleation sites). Even when a weak reverse magnetic field is applied, magnetization inversion occurs from the nucleation sites, resulting in magnetization inversion of the entirety of crystal grains. In particular, an alloy ingot produced through the BM method contains a large number of crystal grains having a major grain size of about some mm, and an alloy ingot produced through the SC method contains a large number of crystal grains having a major grain size of 100 μm or more. By virtue of having such a large grain size, the volume required for magnetization inversion with respect to the total volume of the alloy is large, resulting in very poor magnetic characteristics.
To avoid this, as described above, the alloy ingot is pulverized to have a particle size of about 3 μm, followed by sintering, to thereby produce a magnet. The thus-produced magnet has a crystal grain size of about 5 to about 20 μm, and the low-melting-temperature R-rich phase which becomes a liquid phase during sintering smoothens irregularities of grain boundaries, leading to reduction of nucleation sites, thereby enhancing coercive force. However, the steps of pulverization to sintering involve a considerably high cost. Particularly when the alloy powder is an active R-T-B type magnet alloy powder, measures such as performing the steps of pulverization to sintering in an inert gas atmosphere are required, from the viewpoints of product quality with less variation and greater safety in production steps. Such measures also increase the cost.
Meanwhile, ribbon for R-T-B type bonded magnets produced through the ultra-rapid-cooling method is heated at 500 to 800° C. in accordance with needs, so as to obtain optimal magnetic characteristics. Through heat treatment, ribbon having a crystal grain size in a range from 10 to 100 nm and exhibiting magnetic isotropy is provided. Since the ribbon-form ingot is not practical for use, the ribbon is pulverized to have a particle size of 500 μm or less. A mixture of the powder and a resin is press-molded or injection-molded, to thereby provide an isotropic bonded magnet. There has also been proposed a method for producing bulk isotropic magnets including hot-pressing the ribbon at 700° C. and 1 ton/cm2 (R. W. Lee, Appl. Phys. Lett. 46 (1985), Japanese Patent Application Laid-Open (kokai) No. 60-100402).
However, as compared with the BM method and the SC method, the ultra-rapid-cooling method has low productivity. In addition, a production method of bulk isotropic magnets including hot-pressing requires a high cost.
The alloy ribbon for exchange spring magnets produced through the ultra-rapid-cooling method is also heated at 500 to 800° C. in accordance with needs, so as to obtain optimal magnetic characteristics. Through heat treatment, ribbon having a crystal grain size in a range from 10 to 100 nm and exhibiting magnetic isotropy is provided. Since the ribbon-form ingot is not practical for use, the ribbon is pulverized to have a particle size of 500 μm or less. A mixture of the powder and a resin is press-molded or injection-molded, to thereby provide an isotropic bonded magnet. There has also been disclosed a method for producing bulk isotropic magnets including plasma sintering the ribbon (SPS method) (e.g., Ono, Waki, Fujiki, Shimada, Yamamoto, Sonoda, & Tani, Resume of Lectures, Convention of The Japan Institute of Metals, spring, 2000).
However, as described above, productivity of the ribbon through ultra-rapid-cooling method is low. In addition, a production method of bulk isotropic magnets including plasma sintering involves a significantly high cost.
The present inventors previously improved conventional centrifugal casting methods and devised another solidification process and an apparatus therefor (Japanese Patent Application Laid-Open (kokai) Nos. 08-13078 and 08-332557). Specifically, molten metal is introduced into a rotating mold via a box-like tundish, which is disposed in a reciprocative manner inside the mold and has a plurality of nozzles, whereby the molten metal is deposited and solidified on the inner surface of the rotating mold (this process is called a CC (Centrifugal Casting) process).
In the CC process, molten metal is continuously poured onto an ingot which has already been deposited and solidified. The additionally cast molten metal semi-solidifies while the mold makes one rotation, whereby the rate of solidification can be increased. However, in the production of an alloy of low R content through the CC method, α-Fe which is detrimental to magnetic characteristics and magnet production steps is unavoidably formed due to low cooling rate in a high-temperature zone.
In order to prevent formation of α-Fe in R-T-B type alloy ingots, the present inventors attempted to increase the solidification-cooling rate in the CC process by reducing the deposition rate of a molten metal and previously proposed a centrifugal casting method including sprinkling a molten metal from a rotating tundish and causing the sprinkled molten metal to be deposited on an inner surface of a rotating mold (Japanese Patent Application No. 2000-262605). Through employment of the above method, formation of α-Fe was found to be suppressed. Thereby, a cast alloy ingot of low R content, which enhances magnetic characteristics of produced magnets, can be produced.
However, when the R content decreases, R-rich phase content decreases, possibly resulting in failure to produce sintered magnets of high density and enhanced coercive force. Therefore, it is thought that a minute and uniform dispersion state of the R-rich phase must be attained through more rapid cooling-solidification so as to attain further enhanced magnetic characteristics.
In addition, the thus-produced R-T-B type alloy ingot contains a large number of crystal grains having a major size of 1,000 μm or more, and exhibits very poor magnetic characteristics in an as-cast state. Therefore, further enhancement of the solidification-cooling rate, to thereby reduce the crystal grain size, is deemed necessary.
The present inventors have cried out extensive studies on improvement of conventional centrifugal casting methods and have invented a method which controls the rate of feeding a molten metal and raises the heat transfer efficiency from the cast surface of the alloy ingot which has been deposited and solidified to the inner surface wall of the casting mold.
Thereby, it was confirmed that alloy ingots in which R-rich phase is minutely and uniformly dispersed and which have not been conventionally produced can be obtained, and that sintered magnets produced from the ingots exhibit excellent magnetic characteristics.
In addition, thereby, it is possible to obtain a cast ingot of an R-T-B type alloy having fine crystal gains not available in the past, and it has been confirmed that the cast ingot, as it is, exhibits excellent isotropic magnetic properties.
The present invention has an object of providing a production method for a rare earth alloy ingot and a production method for a rare earth alloy flake which improve the efficiency of heat transfer from the cast surface of the cast ingot to the inner surface wall of the casting mold.
In addition, the present invention has an object of providing a rare earth magnet alloy ingot and a sintered magnet, which have improved magnetic properties.
In addition, the present invention has an object of providing an R-T-B type magnet alloy ingot, an R-T-B type magnet, an R-T-B type magnet alloy flake, an R-T-B type magnet alloy powder, an R-T-B type bonded magnet, an R-T-B type exchange spring magnet alloy ingot, an R-T-B type exchange spring magnet, an R-T-B type exchange magnet alloy powder, and an R-T-B type exchange spring bonded magnet, which have fine crystal grains not available conventionally.